The present invention relates to generally superconducting (FETS), more particularly, to a superconducting FET provided with electrodes of an oxide superconductor which can be produced easily.
There are superconducting devices known as Josephson Junction devices, superconducting transistors, and superconducting quantum interference devices (SQUID). Despite their outstanding characteristic properties such as extremely high operating speed, low power dissipation, and high sensitivity for magnetic field detection, they have been considered to be impracticable because the conventional superconductors become superconducting only at an extremely low temperature close to the liquid helium temperature. A recent breakthrough in this field is the discovery of oxide superconductors that have their temperature higher than the temperature of liquid nitrogen. Superconducting devices based on an oxide superconductor are being put to practical use.
An example of superconducting devices is a superconducting field effect transistor, which is explained in the following with reference to FIG. 1 (sectional view). FIG. 1 shows two sets of identical devices formed on a single substrate. The principle on which this superconducting field effect transistor operates is as follows: A supercurrent induced by the superconducting proximity effect flows from source electrodes 1a, 1b of superconductor to drain electrodes 2a, 2b of superconductor through a semiconductor part 3. This supercurrent is controlled by the application of a gate voltage to gate electrodes 5a, 5b formed on insulators 4a, 4b. In the case where an oxide superconductor is used, it is theoretically known that the distance in which superconducting electrons diffuse from the superconductor to the semiconductor (normal layer) is extremely short. This has led one to believe that the distance between the source and drain electrodes should be shorter than 0.1 .mu.m in the case of superconducting field effect transistor based on an oxide superconductor. In other words, it is generally theorized that superconducting electronics devices (such as a Josphson junction device and superconductor three-terminal device) formed from an oxide superconductor with a high critical temperature should have a channel length corresponding to the coherence length of the oxide superconductor. For example, in the case of a superconducting device formed from a Y--Ba--Cu oxide, which is a typical oxide superconductor, the channel length (or the distance between the two electrodes) should be 0.3-1.4 nm, which is equal to the coherence length of the oxide superconductor. An example of such a superconducting device is reported in Applied Physics Letters, vol. 51, p. 200, 1987. It is a Josephson junction device of Y--Ba--Cu oxide in which the crystal boundary of the oxide constitutes the junction. In other words, the crystal boundary in the narrow part of the Y--Ba--Cu oxide thin film functions as the Josephson junction. The thin film in a loop has two such junction areas to constitute a dc SQUID.
As mentioned above, it has been considered that the Josephson junction and the channel length of a superconducting three-terminal device should have an extremely small dimension, say 1 nm, which equals the coherence length of the oxide superconductor. Unfortunately, it is impossible to form a pattern of oxide superconductor accurately on the order of a nanometer even with the most advanced fine pattern fabrication technique. In other words, there has been no method for producing superconducting devices by forming various fine patterns with high accuracy as designed. The conventional technology does not permit the production of devices with uniform characteristics as designed and the integration of many identical elements on a single substrate. It is very difficult to produce superconducting devices (such as three-terminal devices) which are more complicated in structure than conventional diodes.